The rear window defogger/defroster isn't working on my 1970 144 (hasn't since I bought it). When I pull the knob on the dash out to turn it on, it only lights up (green) if I have it pulled out all the way, not in the low position. Should the light be on in both the low and high positions? What usually causes the failure of the rear defogger? Is it more likely the switch or wiring? I can see there is absolutely no draw on the battery when I turn it on, so it's not even trying to work. The rear window looks to be in nice shape and original. I couldn't locate where the wires from the window grid come from or go.